Electrorecording paper

ABSTRACT

A recording paper having a color-developing layer containing a component capable of being colored or discolored by application of heat which has been made electroconductive by incorporation of an electro-conductive substance. When an electric current is applied to the color-developing layer, the recording paper selectively gives a visible record at a portion of the color-developing layer to which the electric current has been applied.

This invention relates to a recording paper capable of converting anelectric signal directly to a visible image.

Processes for converting electrical signals directly to visible imageshave quickly been diversified in recent years, and processes which arepracticed at present are ink recording, photochemical recording,electrolytic recording, thermal recording and dry discharge recordingprocesses. These processes, however, individually have their merits anddemerits. Recording paper for use in the said processes are desirablysuch that they are usable in the light, can give records high ingradation at a high speed, are stably storable without discoloration orfading, are inexpensive, and require no such operation as exchange ofheads or injection of inks.

As a recording paper which can substantially satisfy the above-mentionedconditions, there has been proposed an electrorecording paper. Thisrecording paper has a color-developing layer containing a componentcapable of being colored or discolored by application of heat. When anelectric current is applied to the color-developing layer, the saidcoloring component is colored due to the heat energy emitted at the timeof application of electric current to give a visible record at aselective portion of the color-developing layer to which the electriccurrent has been applied. Generally, the above-mentioned recording paperis prepared by providing an electroconductive layer on a support, andthen forming a color-developing layer on the surface of theelectroconductive layer.

The electrorecording paper has such characteristics as mentioned belowover a spark destructive recording paper, which is similar in structurethereto and which can be subjected to high speed recording.

That is, the spark destruction recording paper is prepared by forming acolored layer, an electroconductive layer composed of carbon- ormetal-deposited film, and a shielding layer in this order on a support.This recording paper is of such a type that the shielding layer and theelectroconductive layer are removed by discharge to make the lowercolored layer visible. Accordingly, a large amount of pigment isrequired in order to shield the electroconductive layer and the coloredlayer, with the result that the generation of offensive odor and theformation of recording dregs are necessarily brought about due todischarge at the time of recording.

In contrast to this, the aforesaid electrorecording paper is of such atype that when an electric current is applied, a colored or discoloredrecord is formed in the color-developed layer itself, and hence has suchcharacteristics that no colored layer or shielding layer is required,and the generation of offensive odor or fume or the formation ofrecording dregs is scarcely brought about at the time of recording sothat there is no such fear that the equipments used are stained.Moreover, this recording paper is a dry recording paper, and hence hassuch characteristics that it does not require any such preservationvessel as required in the case of wet recording paper and is scarcelyaffected by temperature and humidity, and that it can always besubjected to recording under definite conditions.

Despite the above-mentioned characteristics, the electrorecording paperhas such drawback that when an electric current is applied from arecording needle electrode to the color-developing layer, a partialdischarge takes place between the recording needle electrode and theelectroconductive layer, with the result that there is obtained in thecolor-developing layer only a recorded mark having an area smaller thanthe contact area of said layer with the recording needle electrode, andthus the recorded mark is lowered in visible density.

The present invention aims to improve the said electrorecording paper soas to dismiss such drawbacks as mentioned above.

That is, the present invention provides such electrorecording paper thatthe color-developing layer of the recording paper has been renderedelectroconductive to avoid the occurrence of discharge at the time ofapplication of electric current, thereby giving a record faithful to thetrace of recording needle electrode.

More particularly, the recording paper of the present invention ischaracterized in that an electroconductive substance has been dispersedin the color-developing layer thereof formed by dispersing in a binder acomponent capable of being colored by application of heat.

In the case of the recording paper of the present invention, an electriccurrent flows through the electroconductive substance in thecolor-developing layer, so that the electroconductive substancegenerates heat and the coloring component around the electroconductivesubstance is colored due to the generated heat energy to give a visiblerecord. Accordingly, the addition of the electroconductive substanceresults in such advantages that the recording voltage can be made lowerand, since the electric current flows through the electroconductivesubstance without occurrence of discharge, a record having an areasubstantially identical with the contact area of the recording needlewith the recording paper can be obtained, and thus the recorded image isincreased in visible density.

The recording paper of the present invention is explained in more detailbelow with reference to the accompanying drawings.

FIG. 1 is a drawing showing the fundamental structure of the recordingpaper according to the present invention;

FIG. 2 is a drawing showing the manner of recording an image on therecording paper; and

FIGS. 3 to 5 are drawings showing other structures of the recordingpaper.

In FIGS. 1 and 2, 1 is a support which is a paper, cloth, glass orplastic film; 2 is color-developing layer which develops a color to givea visible image due to the heat energy generated in response to anelectric signal, and has been formed by dispersing in a binder fineparticles of a component capable of being colored by application of heatand an electroconductive substance; 3 is a recording needle electrode;and 4 is another electrode faced to the electrode 3. When a switch 5 isclosed, an electric current flows between the two electrodes through thecolor-developing layer 2 from an alternating or direct current source 6in an amount corresponding to the electric signal desired to berecorded, whereby a colored record 7 is obtained in theelectroconductive portion of the color-developing layer 2.

FIG. 3 shows another structure of the recording paper of the presentinvention in which an electroconductive layer 8 has been disposedbetween the support 1 and the color-developing layer 2. Theelectroconductive layer 8 is composed of a metal-deposited film, cuprousiodide layer or carbon-coated layer, and is made higher in electricconductivity than the color-developing layer 2. Preferable surfaceresistivity of this layer is 10⁴ Ω or less at a temperature of 20° C.and a relative humidity of 65%.

FIG. 4 shows a structure of the present recording paper in which thecolor-developing layer 2 has been formed on an electroconductive support1' such as a carbon fiber- or carbon powder-incorporated paper.Preferable surface resistivity of the support 1' in 10⁴ Ω or less.

FIG. 5 shows a structure which is fundamentally identical with thatshown in FIG. 3 but is different therefrom in that a secondcolor-developing layer 2' has been formed between the color-developinglayer 2 and the electroconductive layer 8. The color-developing layer 2'contains no electroconductive substance, and is composed of fineparticles dispersed in a binder of a component capable of being coloredby application of heat.

In the color-developing layer 2, to which no electroconductive substancehas been added, the absolute amount of color-forming component tends tobecome small, so that there are some cases where the density of recordedimage is not satisfactory. The recording paper of the structure shown inFIG. 5 has, below the color-developing layer 2, the secondcolor-developing layer 2' containing no electroconductive substance, andhence has such effect that the color-developing layer 2' develops acolor due to the electric current flowed from the color-developing layer2 to the electroconductive layer 8 through the color-developing layer2', whereby the density of recorded image is increased.

The materials used in the present invention, i.e., the thermorecordingmaterial capable of forming a color by application of heat, theelectroconductive substance, and the binder, are explained below.

1. THERMORECORDING MATERIAL

This material develops a color by utilization of the heat energygenerated at the time of application of electric current. As suchmaterial, there is one which has utilized the development of color dueto formation of a reaction compound of an electron donor with anelectron acceptor. For example, ferric stearate as the electron donorand pyrogallol as the electron acceptor are independently dispersed in abinder, and the binder is softened by application of heat to melt theferric stearate, whereby a record can be obtained. The record obtainedin this case is a dark brown record. As such thermorecording material,there is used the combination of a leuco body of such a triphenylmethanetype dye as Crystal Violet Lactone or a leuco body of a fluoran type dyewith an organic acid or a phenolic acidic substance, or the combinationof an organic compound having metal ions with an organic spot reagent.There is adopted the procedure that the said dye and organic acid, orsaid organometallic compound and organic spot reagent, are independentlydispersed in a binder.

a. Color-forming dye

Generally, the color-forming dye used is a leuco body oftriphenylmethane type dye represented by the below-mentioned generalformula (I) or a leuco body of fluoran type dye represented by thebelow-mentioned general formula (II). ##STR1## wherein R_(x), R_(y) andR_(z) are individually a hydrogen or halogen atom, or a hydroxyl, alkyl,nitro, amino, dialkylamino, monoalkylamino or aryl group; and Z is anatom necessary to form a heterocyclic ring, and is O. .[. or S.].

Concrete examples of the above-mentioned compounds are as shown below.

Compounds of the formula (I):

3,3-bis(p-dimethylaminophenyl)-phthalide

3,3-Bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (Crystal VioletLactone)

3,3-Bis(p-dimethylaminophenyl)-6-aminophthalide

3,3-Bis(p-dimethylaminophenyl)-6-nitrophthalide

3,3-Bis(p-dibutylaminophenyl)-phthalide

3,3-Bis(p-dimethylaminophenyl)-4,5,6,7-tertrachlorophthalide

Compounds of the formula (II):

3-dimethylamino-6-methoxyfluoran

7-Acetamino-3-dimethylaminofluoran

3-Dimethylamino-5,7-dimethylfluoran

3-Diethylamino-5,7-dimethylfluoran

3,6-Bis-β-methoxyethoxyfluoran

3,6-Bis-β-cyanoethoxyfluoran

Other lactam compounds:

9-p-Nitroanilino-3,6-bis(diethylamino)-9-xanthenyl-o-benzoic acid lactam(Rhodamin B lactam)

9-p-Nitroanilino-3,6-bis(dimethylamino)-9-thioxanthenyl-o-benzoic acidlactam

The above-mentioned dye bases are effectively used. They are scarcelysoluble in water, and can be pulverized to fine particles of less than10 microns in size.

b. Coupler

Preferable as the coupler capable of forming a dye by chemical reactionwith the color-forming dye mentioned in the preceding item (a) is aphenolic compound or an organic acid. It is desirable that the saidcompound or acid is solid at room temperature and can liquefy orvaporize at above 70° C.

i. Examples of the phenolic compound are as follows:

3,5-Xylenol, thymol, 4-tert-butylphenol, 4-hydroxyphenoxide,methyl-4-hydroxybenzoate, 4-hydroxyacetophenone, α-naphthol, β-naphthol,catechol, hydroquinone, resorcinol, 4-tert-octylcatechol,4,4'-sec-butylidenediphenol, 2,2'-dihydroxydiphenyl,2,2'-methylenebis(4-methyl-6-tert-butylphenol),2,2'-bis(4-hydroxyphenyl)-propane,4,4'-isopropylidenebis-(2-tert-butylphenol),4,4'-sec-butylidenediphenol, pyrogallol and 4,4'-isopropylidenediphenol.

ii. Examples of the organic acid are as follows:

Stearic, gallic, benzoic, salicylic, succinic, 1-hydroxy-2-naphthoic,2-hydroxy-p-toluic, o-hydroxybenzoic, m-hydroxybenzoic, p-hydroxybenzoicand 4-hydroxyphthalic acids.

c. Organic spot reagent and organic metal salt

The organic spot reagent referred to herein is a compound capable ofbeing colored or discolored by reaction with metal ions. At present, alarge number of such compounds are known. In the present invention,however, there is utilized the phenomenon that at least one of theorganic spot reagent and the metal used in combination therewith ismelted due to the heat energy generated at the time of application ofelectric current and the two react with each other to form a color. Itis therefore desirable that one of the two which is lower in meltingpoint is solid at below 70° C. and has a melting point of 150° C. orless. In this respect, metallic soap is most preferable as the organicmetal salt. Combinations of the organic spot reagent and the metal areas follows:

i. Organic spot reagent: Metal:

Diphenylthiocarbazide: Cu, Fe, Mg or Hg

Dimethylglyoxime: Cu, Fe or Ni

Benzoinoxime: Cu

8-Hydroxyquinoline: Cd, Cu, Fe, Pb, Mn, Ni or Zn

Dinitrophenylcarbazide: Cd

Rhodanine: Cu or Hg

Diphenylthiocarbazone: Cu, Ba, Co, Fe, Pb, Hg or Zn

Diphenylcarbazone: Co, Cu, Pb, Mg, Mn, Hg, Ni or Zn

Dithiooxamide: Co, Cu, Pb or Ni

2-Mercapto-4-phenylthiazole: Co or Pb

3,5-Dimethylpyrazole: Co

α-Naphthylamine-dithiocarbamic acid: Co or Fe

Benzoinoxime: Cu

Benzidine: Cu, Pb or Mn

p-Dimethylaminobenzylidene rhodanine: Cu, Fe, Mg or Hg

Salicylaldoxime: Cu or Pb

Triphenyl thiophosphate: Ni

p,p'-Tetramethyl-diaminodiphenylmethane: Pb or Mn

Anthranilic acid: Zn

Diphenylbenzine: Zn

Catechol: Fe

Gallic acid: Fe

Dihydroxynaphthalene: Fe

Alizarine: Cu

Quinalizarin: Cu

ii. Organic metal salt:

As the organic metal salt, a metallic soap having the aforesaid metalions is effective. Typical examples of the combinations of organic sportreagents with organic metal salts, and color tones of colors developedby use of said combinations, are shown below.

Organic spot reagent: Organic metal salt (Color tone of developed color)

Dimethylglyoxime: Nickel stearate (Pink)

Benzoinoxime: Copper myristate (Yellowish green)

Dithiooxamide: Nickel stearate (Purple)

8-Hydroxyquinoline: Iron oleate (Black)

Gallic acid: Ferric stearate (Black)

Alizarin: Copper oleate (Purplish red)

Quinalizarin: Copper oleate (Purplish red)

Diphenylcarbazone: Copper stearate (Red)

Diphenylcarbazone: Cadmium stearate (Red)

Diphenylcarbazone: Copper myristate (Purple)

Diphenylcarbazone: Zinc palmitate (Deep red)

Diphenylthiocarbazide: Mercuric stearate (Purple)

Diphenylthiocarbazide: Lead myristate (Deep red)

d. It has been found that .[.s.]. .Iadd.a .Iaddend.substance, which hasheretofore been known as redox indicator, forms a color in the vicinityof recording needle at the time of application of electric current, andit has been confirmed that said substance is effective as acolor-forming material for use in electrorecording paper. The redoxindicator referred to herein is a substance which is colored ordiscolored due to oxidation with the heat generated at the time ofapplication of electric current, and is a leuco body is preferable inview of the whiteness of the background. Examples of the redoxindicators used in the present invention, and color tones of colorsdeveloped by said indicators, are shown below.

Leucoethyl Nile Blue (Blue)

Leucomethyl Capryl Blue (Blue)

Leucotoluidine Blue (Purple)

Leucodiphenylamine (Purple)

Leuco-N-methyldiphenylamine-p-sulfonic acid (Reddish purple)

Leucophenylanthranilic acid (Reddish purple)

Triphenyltetrazolium chloride (Red)

Methylviologen (Purple)

Leucosafranine T (Red)

Leucoindigosulfonic acid (Blue)

Leucophenosafranine (Red)

Leucomethylene Blue (Blue)

Leucodiphenylbenzidine (Purple)

Leucoerioglucine A (Yellowish green to Red)

Leuco-p-nitrodiphenylamine (Purple)

Leucodiphenylamine-o,o'-diphenylcarboxylic acid (Bluish purple)

2. ELECTROCONDUCTIVE SUBSTANCE

The electroconductive substance used in the present invention is amaterial which is incorporated into the color-developing layer ofrecorder paper to impart electroconductivity to said layer. In order toobtain a record faithful to the trace of recording needle without anydischarge at the time of recording and at a voltage as low as possible,the surface resistivity of the color-developing layer is required to be10⁷ Ω or less at a temperature of 20° C. and a relative humidity of 65%.It is desirable that the electroconductive substance, which is to beincorporated into the color-developing layer, is .[.scarely.]..Iadd.scarcely .Iaddend.affected in electric conductivity by temperatureand humidity, and is transparent and white or pale-colored so that theresulting recorded image can be enhanced in contrast. In order to lowerthe voltage at the time of recording to make it possible to carry outthe recording at an alternating current voltage of less than 1.5 KV.,the resistivity of the electroconductive substance is desirably 10⁴ Ωcm. or less. As such electroconductive substance, there is used a knownsemiconductor which is white or pale-colored, or a material prepared bycoating an electroconductive film on the surface of a powder comprisinga white or pale-colored substrate.

Electroconductive substances preferable for use in the recording papersof the present invention are shown below.

a. Cuprous iodide

Cuprous iodide is obtained as a transparent or pale yellow compound, andhence is most preferable as the .[.electroncudctive.]..Iadd.electroconductive .Iaddend.substance used in the presentinvention. It is desirable that a recording paper satisfies such demandthat it does not give an impression of specific paper but isundistinguishable in appearance from ordinary paper. When cuprous iodideis used as the electroconductive substance to be incorporated into thecolor-developing layer, it is possible to obtain a recording paper whichcan satisfy the above-mentioned demand. Even when an image recorded onthe present recording paper containing cuprous iodide as theelectroconductive substance is subjected to copying by use of atransmitted light, the cuprous iodide does not bring about any injury atall. In case the cuprous iodide is used to form the electroconductivelayer 8 in the case where the recording paper has such a structure asshown in FIGS. 3 or 5, an image recorded on the recording paper can bereproduced by utilization of a transmitted light.

Cuprous iodide is prepared, for example, in the following manner:

500 Cubic centimeters of a 30% aqueous potassium iodide solution ismixed under stirring with 500 cc. of a 15% aqueous copper sulfatesolution to deposit a precipitate. This precipitate is separated bycentrifuge and then washed 5 to 6 times with water, whereby white powdercuprous iodide to obtained in a high yield. This powder is dried andmeasured in resistivity at room temperature to obtain a value of 2 to 4Ω cm. Cuprous iodide is scarely soluble in water and organic solvents.

b. Tin dioxide

Tin dioxide is a white powder insoluble in water and organic solvents.In the examples shown later was used tin dioxide having a resistivity of2.3 × 10³ Ω cm.

This tin dioxide can be lowered in resistivity when antimony is diffusedtherein. For example, 50 g. of the above-mentioned tin dioxide is mixedwith 20 cc. of a 10% methanol solution of antimony trichloride, and theresulting mixture is dried at 60° C. and then calcined for about 30minutes in an electric furnace at 300° to 400° C. to obtain tin dioxide.The thus obtained tin dioxide having antimony diffused therein is a paleblue powder and has a resistivity of 20 to 40 Ω cm.

c. Silver iodide

An aqueous potassium iodide solution is mixed with an aqueous silvernitrate solution, and the resulting mixed solution is stirred to deposita pale yellow precipitate. This precipitate is recovered by filtration,washed 2 to 3 times with water, and then dried to obtain silver iodidehaving a resistivity of 7.8 × 10³ Ω cm.

d. Other electroconductive substance:

A transparent and white or pale-colored substance, which itself has noelectric conductivity but is pulverizable into fine particles of lessthan about 20 microns, may also be used by coating on the surfacethereof a "Nesa" (SnO₂ + Sb) film or a zinc- or aluminum-deposited film.

An example of procedure for preparing silica gel coated with a "Nesa"film is explained below.

To a mixed solution comprising 150 cc. of water, 50 cc. of methanol and20 cc. of 35% hydrochloric acid are added 100 g. of tin tetrachloride, 3g. of antimony trichloride and 500 g. of silica gel, and the resultingmixture is pulverized in a ball mill for 24 hours to form a dispersion.This dispersion is dried at 100° C. and then calcined for 10 minutes inan electric furnace at 600° C. The calcination product is washed with a3:1 mixture of water and methanol to remove excess tin tetrachloride andantimony trichloride, and then dried to obtain a white fine powder ofsilica gel coated with "Nesa" film. The thus obtained powder has aresistivity of 3.5 × 10³ Ω cm.

Alternatively, a white substance such as zinc oxide, titanium dioxide,magnesium carbonate, calcium oxide or lead carbonate is pulverized intoparticles of 5 to 10 microns, and a vapor of zinc or aluminum isdeposited on the particles with stirring, whereby a gray or white finepowder having a resistivity of 2 × 10⁻¹ Ω cm. to 10.sup. 4 Ω cm. isobtained. The thus obtained powder may also be used as theelectroconductive substance.

3. BINDER

In order to disperse in the state of fine particles the color-formingdye, coupler, organic spot reagent and organic metal salt used in thecolor-developing layer, and/or the electroconductive substance, and toimpart bonding ability thereto, there is used a binder. Since most ofthe above-mentioned color-forming dye, coupler, organic spot reagent andelectroconductive substance are water-insoluble, the use of awater-soluble binder is effective. Further, the water-soluble substancehas such characteristic that it is easily handled and treated at thetime .[.or.]. .Iadd.of .Iaddend.production of recording paper.

i. Water-soluble binder:

Examples of the water-soluble binder include hydroxyethyl cellulose,carboxymethyl cellulose, methoxy cellulose, polyvinyl alcohol, polyvinylpyrrolidone, polyacrylamide, polyacrylic acid, gelatin and starch.

As a film-forming binder which is water-soluble and electroconductive,there is a high molecular electrolyte. This electrolyte contributes tothe increase in electric conductivity of electrorecording paper, and iseffectively used for the purpose of lowering the voltage at the time ofrecording. The high molecular electrolyte includes such cationic andanionic electrolytes as mentioned below.

a. Cationic high molecular electrolyte:

This electrolyte has a functional group of the formula, ##STR2## whereinR₁, R₂ and R₃ are individually an alkyl, allyl, aryl, acyl or alkylaminogroup; and X is a halogen atom.

Concrete examples of the electrolyte are as follows:

Poly-2-acryloxyethyl dimethylsulfonium chloride ##STR3## Polyglycidyltributylphosphonium chloride ##STR4##

b. Anionic high molecular electrolyte:

This electrolyte has a functional group of the formula, ##STR5## whereinM is an alkali metal or an alkaline earth metal.

Concrete examples of the electrolyte are as follows:

Sodium .[.polymethylacrylate.]. .Iadd.polymethacrylate .Iaddend.##STR6## Sodium polystyrenesulfonate ##STR7## Sodium polyvinylsulfonate##STR8## Potassium polyvinylphosphate ##STR9## ii. water-insolublebinder:

As the binder, an organic solvent-soluble binder may also be used. Incase a binary system comprising, for example, a color-forming dye and acoupler, is used as the thermorecording material of the color-developinglayer, the two components should individually be dispersed in the formof fine particles into the binder. If either one of the two componentsis dissolved in a solvent used to dissolve the binder, a color formationreaction to deprive its function as a recording medium takes place atthe time of mixing of the two. Accordingly, solvents for the bindermentioned herein are necessarily limited depending on the kind ofcolor-forming components.

Concrete examples of the water-insoluble binder are natural rubber,synthetic rubbers, chlorinated rubbers, alkyd resins, styrene-butadienecopolymers, polybutyl methacrylate, low molecular weight ethylenepolymers, low molecular weight styrene polymers, polyvinyl butyral,phenolic resins and nitrocellulose.

The present invention is illustrated in detail below.

Examples 1 to 6 show recording papers of the structure shown in FIG. 1.

EXAMPLE 1

100 Parts by weight of a 5 % toluene solution of a styrene-butadienecopolymer (styrene:butadiene = 85:15) was mixed with 25 parts by weightof ferric stearate, and the resulting mixture was pulverized in a 500ml. ball mill for 24 hours to prepare a dispersion (liquid A). In theliquid A, the ferric stearate had scarely dissolved and had dispersed inthe form of fine particles of less than 5 microns.

On the other hand, 100 parts by weight of a 5% toluene solution of astyrene-butadiene copolymer was mixed with 25 parts by weight of gallicacid, and the resulting mixture was pulverized in a 500 ml. ball millfor 24 hours to prepare a dispersion (liquid B). In the liquid B, thegallic acid had scarcely dissolved and had dispersed in the form of fineparticles of less than 5 microns.

Likewise, 100 parts by weight of a 5 % toluene solution of astyrene-butadiene copolymer was mixed with 100 parts by weight ofcuprous iodide to prepare a dispersion (liquid C).

.[.Subsequnelty,.]..Iadd.Subsequently, .Iaddend.15 parts by weight ofthe liquid A, 15 parts by weight of the liquid B and 100 parts by weightof the liquid C were homogeneously mixed together by means of a mixer toform a liquid D. The liquid D was coated on a white paper having athickness of about 60 microns by use of a wire bar, which had been soadjusted as to form a film having a dry thickness of 15 microns, and wasthen dried to obtain an electrorecording paper. The thus obtainedrecording paper had a pale brown ground having a reflective density of D= 0.05, but was substantially white when observed visually. The surfaceresistivity of the recording paper was 2.35 × 10⁴ Ω. This recordingpaper was subjected to recording in the manner shown in FIG. 2, using asthe recording needle .[.electroce.]. .Iadd.electrode .Iaddend.3 atungsten wire of 0.5 mm. in diameter and running the wire at a rate of540 mm/sec., while applying an alternating current voltage of 300 V. toobtain a dark purple record having a reflective density of D = 0.76.This recording paper could be subjected to copying using a transmittedlight.

In the next place, the aforesaid liquids A and B were mixed with eachother in a weight ratio of 1:1 to prepare a liquid E. The liquid E wasmixed with 100 parts by weight of the aforesaid liquid C, and theresulting mixture was coated on a white paper in the same manner asmentioned previously to obtain a recording paper. This recording paperwas subjected to recording under the same conditions as mentioned above.The relation between the amount of the liquid E per 100 parts by weightof the liquid C, the surface resistivity of the recording paper, and thereflective density of the recorded image formed on the recording paperwas as shown in the following table:

    ______________________________________                                        Amount of liquid E                                                                         Surface        Reflective                                        (parts by weight)                                                                          resistivity (Ω)                                                                        density (D)                                       ______________________________________                                        5            7.16 × 10.sup.3                                                                        0.13                                              10           8.80 × 10.sup.3                                                                        0.23                                              20           1.52 × 10.sup.4                                                                        0.52                                              30           2.35 × 10.sup.4                                                                        0.76                                              40           2.82 × 10.sup.4                                                                        0.74                                              50           4.56 × 10.sup.4                                                                        0.76                                              ______________________________________                                    

EXAMPLE 2

A mixture comprising 100 parts by weight of titanium dioxide and 100parts by weight of acetone was pulverized in a 500 ml. ball mill for 24hours, filtered and then dried to obtain a fine titanium dioxide powderof less than 5 microns in particle size. Onto this titanium dioxidepowder, a vapor of aluminum was deposited by vacuum evaporation under 3× 10⁻⁵ torr with shaking, whereby a fine titanium dioxide powder havingan aluminum coating was obtained. The specific resistance of thistitanium dioxide powder was 5 Ω cm.

100 Parts by weight of the fine titanium dioxide powder, 30 parts byweight of the liquid A used in Example 1, 30 parts by weight of theliquid B used in Example 1, and 100 parts by weight of a 5 % toluenesolution of a styrene-butadiene copolymer were mixed together withstirring. The resulting mixed liquid was coated to a thickness of 15microns on a white paper having a thickness of 60 microns, and was thendried to obtain a recording paper.

The thus obtained recording paper was pale gray-colored and had asurface resistivity of 2.8 × 10⁴ Ω. This recording paper was subjectedto recording under the same conditions as in Example 1, whereby arecorded image having a reflective density of D = 0.82 was obtained.

EXAMPLE 3

100 Parts by weight of cuprous iodide was mixed with 100 parts by weightof a 5 % aqueous solution of polyvinyl alcohol ("PVA-205" produced byKuraray Co.), and the resulting mixture was pulverized in a 500 ml. ballmill for 48 hours to prepare a dispersion (liquid A).

In the next place, 200 parts by weight of a 10 % aqueous polyvinylalcohol solution was mixed with 35 parts by weight of3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide as acolor-forming dye, and the resulting mixture was pulverized in a 500 ml.ball mill for 48 hours to prepare a dispersion (liquid B). In the liquidB, the color-forming dye had scarcely dissolved and had dispersed in theform of fine particles of less than 5 microns.

Separately, 200 parts by weight of a 10 % aqueous polyvinyl alcoholsolution was mixed with 35 parts by weight of4,4'-isopropylidenediphenol (Bis-phenol A) as a coupler, and theresulting mixture was pulverized in a 500 ml. ball mill for 48 hours toprepare a liquid C. In the liquid C, the coupler had dispersed in theform of fine particles of less than 5 microns.

Subsequently, 10 parts by weight of the liquid B and 100 parts by weightof the liquid C were stirred and mixed together by means of a mixer toprepare a liquid D. In the liquid D, the color-forming dye and thecoupler had individually dispersed in the form of fine particles, and itmay be said that there was no direct contact between the two particles.

100 Parts by weight of the aforesaid liquid A and 30 parts by weight ofthe liquid D were stirred and mixed together by means of a mixer. Theresulting mixture was coated on the surface of a paper by use of a wirebar, which had been so adjusted as to form a film having a thickness of15 microns, and was then dried to obtain an electrorecording paper.

This recording paper was subjected to recording in the manner as shownin FIG. 2, using as the recording needle electrode 3 a tungsten wire of0.25 mm. in diameter and running the wire at a rate of 540 mm/sec. whileapplying an alternating current voltage of 300 V., whereby a brilliantblue record having a reflective density of 0.86 was obtained.

EXAMPLE 4

60 Parts by weight of the liquid D used in Example 3, 100 parts byweight of the fine silica gel powder coated with a "Nesa" (SnO₂ + Sb)transparent film which was stated in item (d) of the paragraphElectroconductive substance and 100 parts by weight of a 5 % aqueouspolyvinyl alcohol solution were mixed together by means of a mixer. Theresulting mixture was coated on a white paper so as to form a filmhaving a dry thickness of 15 microns, and was then dried to obtain anelectrorecording paper. The surface resistivity of the thus obtainedrecording paper was 2.5 × 10⁶ Ω. This recording paper was subjected torecording in the manner shown in FIG. 2 while applying an alternatingcurrent voltage of 1.5 KV., whereby a blue recorded image having areflective density of D = 0.67 was obtained.

EXAMPLE 5

100 Parts by weight of cuprous iodide, 10 parts by weight ofLuecomethylene Blue, and 130 parts by weight of a 1 % aqueous gelatinsolution were mixed together, and the resulting mixture was pulverizedin a ball mill for 24 hours to prepare a dispersion. This dispersion wascoated on a white paper and then dried to obtain a white recordingpaper. An electric current was applied to the thus obtained recordingpaper, whereby a pale blue record was obtained.

EXAMPLE 6

100 Parts by weight of antimony-diffused tin dioxide, 10 parts by weightof Leucomethylene Blue, and 130 parts by weight of a 1 % aqueous gelatinsolution were mixed together, and the resulting mixture was pulverizedin a ball mill for 24 hours to prepare a dispersion. This dispersion wascoated on a white paper so as to form a film having a thickness of 15microns and was then dried to obtain a pale blue recording paper. Anelectric current was applied to the thus obtained recording paper,whereby a record having a contrast of 0.32 was obtained.

Examples 7 to 10 set forth below illustrate the cases where theelectroconductive layers 8 shown in FIGS. 3 and 5 are formed.

EXAMPLE 7

This example illustrates a procedure of depositing cuprous iodidedirectly on a support by vacuum evaporation, in which the curpous iodideused was a first grade commerically available product.

An art paper as a support was adhered to the interior of a bell jar. Onthe other hand, about 0.5 g. of the cuprous iodide was charged into aquartz glass pot, and a tungsten wire of 0.5 mm. in diameter was woundaround the pot. When the vapor pressure inside the bell jar had.[.becomes.]. .Iadd.become .Iaddend.7 × 10⁻⁵ .[.toor,.]. .Iadd.torr,.Iaddend.an electric current of 10 to 15 mA was applied to the tungstenwire and, after about 10 minutes, the electric current was shut off tocomplete the vacuum evaporation. The resulting cuprous iodide-depositedpaper was white and had a surface resistivity of 5 × 10³ to 2 × 10⁴ Ω.When exposed to ultraviolet rays of fluorescent lamp and the like, thepaper was yellowed but showed no change in resistivity.

EXAMPLE 8

Fine particles of copper were charged into a tungsten basket and weredeposited by vacuum evaporation on an art paper under a pressure of 5 ×10⁻⁵ torr in the same manner as in Example 7. The surface resistivity ofthe copper-deposited paper was 2 to 3 Ω. This paper was placed in adesiccator containing fine particles of iodine, and was sucked with anaspirator for 15 to 45 minutes, whereby the copper on the paper surfacereacted with the iodine to form cuprous iodide, and the red surface ofthe paper gradually became white. When the suction was furthercontinued, the iodine became excess, and the paper came to be yellowed.The yellowed paper was allowed to stand in air, whereby the excessiodine was gradually released, and the paper became a white paper again.The surface resistivity of the thus obtained paper was 10³ to 4 × 10³ Ω.

EXAMPLE 9

100 Cubic centimeters of a 4 % aqueous solution of polyvinyl alcohol("PVA 205" produced by Kuraray Co.) was mixed with 100 g. of cuprousiodide, and the resulting mixture was pulverized in a 500 ml. ball millfor 24 hours to form a dispersion. This dispersion was coated on an artpaper by means of a wire bar, which had been so adjusted as to form afilm having a thickness of 15 microns, and was then dried to obtain acoated paper. The surface resistivity of this coated paper was (2 to 5)× 10³ Ω.

Using other water-soluble binders such as hydroxyethyl cellulose,gelatin, carboxymethyl cellulose, polyacrylamide, polyethylene oxide andpolyvinyl pyrrolidone, coated papers were prepared in the same manner asabove. The thus prepared coated papers were substantially identical insurface resistivity with the coated paper obtained in the above.

EXAMPLE 10

100 Cubic centimeters of a 10 % toluene solution of an 85:15 copolymerof styrene and butadiene was mixed with 100 g. of cuprous iodide, andthe resulting mixture was pulverized in a 500 ml. ball mill for 24 hoursto form a dispersion. This dispersion was coated on an art paper bymeans of a wire bar, which had been so adjusted as to form a film havinga thickness of 15 microns, and was then dried to obtain a coated paper.The surface resistivity of this coated paper was (0.5 to 1) × 10⁴ Ω.

EXAMPLE 11

A mixture comprising 25 parts by weight of nickel stearate and 100 partsby weight of a 3 % aqueous polyacrylamide solution was pulverized in aball mill to prepare a dispersion (a). On the other hand, a mixturecomprising 25 parts by weight of dimethyl glyoxime and 100 parts byweight of a 3 % aqueous polyacrylamide solution was pulverized in a ballmill to prepare a dispersion (b). 30 Parts by weight of the dispersion(a), 30 parts by weight of the dispersion (b), 100 parts by weight of afine silica gel powder coated with a "Nesa" film, and 100 parts byweight of a 3 % aqueous polyacrylamide solution were mixed together. Theresulting mixture was coated to a dry film thickness of 8 microns on theelectroconductive cuprous iodide layer obtained in Example 1, and wasthen dried to obtain a pale blue recording paper. This recording paperwas subjected to recording under the same conditions as in Example 1,while applying an alternating current voltage of 100 V., whereby a pinkrecord having a reflective density of D = 0.63 was obtained.

    ______________________________________                                        Example 12                                                                    ______________________________________                                                  5 % Aqueous hydroxyethyl                                            A         cellulose solution                                                                              200   parts by weight                                       3,6-Dimethylfluoran                                                                              35   "                                                     5% Aqueous hydroxyethyl                                             B         cellulose solution                                                                              200   parts by weight                                       1-Hydroxybenzoic acid                                                                            35   "                                                     5 % Aqueous hydroxyethyl                                            C         cellulose solution                                                                              100   parts by weight                                       Cuprous iodide    100   "                                           ______________________________________                                    

The above-mentioned mixtures A, B and C were individually pulverized ina ball mill for 24 hours to prepare liquids A, B and C, respectively. 5Parts by weight of the liquid A, 50 parts by weight of the liquid B, 200parts of the liquid C were mixed together under stirring by means of amixer. The resulting mixture was coated on the cuprous iodide layer ofExample 9 so as to form a film having a dry thickness of 8 microns andwas then dried to obtain a white recording paper. This recording paperwas subjected to recording in the same manner as in Example 1, whereby arecord having a reflective density of D = 0.83 was obtained.

EXAMPLE 13

The liquid D of Example 1 was coated on an aluminum-deposited paper soas to form a film having a dry thickness of 10 microns and was thendried to obtain a pale brown recording paper. This recording paper wassubjected to recording in the same manner as in Example 1, whileapplying an alternating current voltage of 40 V., whereby a black recordwas obtained. When the voltage was increased to 150 V. or more, sparkdestruction took place to generate fumes as well as to ooze the record.

EXAMPLE 14

100 Parts by weight of amorphous carbon, 10 parts by weight of a lowstyrene polymer and 100 parts by weight of toluene were mixed together,and the resulting mixture was pulverized in a ball mill for 24 hours toprepare a dispersion. This dispersion was coated on a white paper so asto form a film having a dry thickness of 10 microns, and was then driedto form an electroconductive layer. The surface resistivity of thiselectroconductive layer was 250 to 500 Ω.

Subsequently, a mixture comprising 10 parts by weight of LeucomalachiteGreen, 100 parts by weight of silver iodide, 5 parts by weight of sodiumpolystyrenesulfonate and 130 parts by weight of water was pulverized ina ball mill for 24 hours to prepare a dispersion. This dispersion wascoated on the aforesaid electroconductive layer so as to form a filmhaving a dry thickness of 10 microns, and was then dried to obtain apale yellow recording paper.

The thus obtained recording paper was subjected to recording, whileapplying a direct current voltage of 70 V., whereby a green record wasobtained.

    ______________________________________                                        Example 15                                                                    ______________________________________                                                  10 % Aqueous polyvinyl                                              A         alcohol solution  100   parts by weight                                       Copper palmitate   25   "                                                     10 % Aqueous polyvinyl                                              B         alcohol solution  100   parts by weight                                       Diphenylcarbazone  25   "                                                     5 % Aqueous polyvinyl                                               C         alcohol solution  100   parts by weight                                       Cuprous iodide    100   "                                           ______________________________________                                    

The above-mentioned mixtures A, B and C were individually pulverized ina ball mill for 24 hours to prepare liquids A, B and C, respectively. 15Parts by weight of the liquid A, 15 parts by weight of the liquid B and100 parts by weight of the liquid C were mixed together under stirringby means of a mixer. The resulting mixture was coated to film thicknessof 10 microns on an electroconductive paper containing 40 % of carbonfibers and having a surface resistivity of 250 Ω, and was then dried toobtain a recording paper. This recording paper was subjected torecording while applying a voltage of 100 V., whereby a reddish purplerecord was obtained.

EXAMPLE 16

This example illustrates a recording paper having the structure shown inFIG. 5.

Between the electroconductive cuprous iodide layer and thecolor-developing layer containing 3,6-dimethylfuoran andp-hydroxybenzoic acid as color-forming components, which layers wereformed in Example 12, a 1:10 mixture of the liquid A and B prepared inExample 12 was disposed so as to form a film having a thickness of 3microns to obtain a white recording paper having the color-developinglayer 2' shown in FIG. 5. This recording paper was subjected torecording under the same conditions as in Example 12 to give a brilliantred record having a reflective density of D = 1.21.

What we claim is:
 1. An electrorecording paper having a color-developinglayer containing a component capable of changing color in response toapplied heat, said color-developing layer comprising a heat coloringcomponent and an electroconductive substance, both of which aredispersed in the form of fine particles in a binder; saidelectroconductive substance being substantially light reflective,electron conductive and having a specific resistance not greater than10⁴ Ω cm and selected from the group consisting of cuprous iodide, tindioxide.[.,.]. .Iadd.and .Iaddend.silver-iodide.[., and antimony.].; andsaid color-developing layer having a .[.surfacee.]. .Iadd.surface.Iaddend.resistivity of not greater than 10⁷ Ω .[.cm..]. at atemperature of 20° C. and at a relative humidity of 65%; said layerbeing capable of recording by changing color in response to heat appliedby means of an applied electric current.
 2. An electrorecording paperaccording to claim 1, wherein said electroconductive substance iscuprous iodide.
 3. An electrorecording paper according to claim 1,wherein said color-developing layer is in contact with a supportingmember.
 4. A recording paper according to claim 1, wherein the componentcapable of being colored by application of heat is composed of a leucobody of a triphenylmethane type dye or a leuco body of a fluoran typedye and a phenolic substance or an organic acid.
 5. A recording paperaccording to claim 1, wherein the component capable of being colored byapplication of heat is composed of a metal salt and an organic spotreagent, which reacts with metal ions of said salt to form a visuallyobservable reaction product, one of said two substances which is lowerin melting point having a melting point of 70° to 150° C.
 6. A recordingpaper according to claim 1, wherein the component capable of beingcolored by application of heat is a redox indicator. .[.7. A recordingpaper according to claim 1, wherein the electroconductive substance isantimony-containing tin dioxide..]. .[.8. A recording paper according toclaim 1, wherein the electroconductive substance is silica gel coatedwith antimony-containing tin dioxide..]. .[.9. A recording paperaccording to claim 1, wherein the electroconductive substance is coatedwith a metal film..].
 10. A recording paper according to claim 3,wherein an electroconductive layer is disposed between the supportingmember and the color-developing layer.
 11. A recording paper accordingto claim 10, wherein the electroconductive layer is a layer composed ofa .[.support and a.]. metal, which has been deposited on the support byvacuum evaporation.
 12. A recording paper according to claim 10, whereinthe electroconductive layer is composed of cuprous iodide.
 13. Arecording paper according to claim 12, wherein the electroconductivelayer composed of cuprous iodide .[. is a layer comprising a support andcuprous iodide, which.]. has been deposited on the support by vacuumevaporation.
 14. A recording paper according to claim 12, wherein theelectroconductive layer composed of cuprous iodide is a layer comprisingparticles of cuprous iodide which have been bonded by use of a binder.15. A recording paper according to claim 10, wherein theelectroconductive layer is a layer comprising carbon particles whichhave been bonded by use of a binder. A recording paper according toclaim 3, wherein the support is an electroconductive material.
 17. Arecording paper according to claim 3, wherein a second color-developinglayer formed by dispersing in a binder fine particles of a componentcapable of being colored by application of heat is disposed between thecolor-developing layer and the support.
 18. A recording paper accordingto claim 17, wherein an electroconductive layer is disposed between thesecond color-developing layer and the support.